So you want to "burn fat?"

Potential for Improving Endurance Performance through Substrate Conservation

We know skeletal muscle needs ATP to contract and there are different pathways from which ATP can be synthesized. During endurance exercise, inevitably, some combination of glucose/glycogen and fatty acids will be used for substrate. Utilizing fatty acids for ATP synthesis could be beneficial for endurance athletes because humans have a virtually unlimited supply fatty acids, whereas stored carbohydrate is relatively limited - demonstrated below:

135,000 Cal from stored fat is roughly enough energy for a 65 kg person to run 2000 km (R. Margaria et al, 1963).
A decreased reliance on carbohydrate and a subsequent increased reliance on fatty acids during exercise should help to "spare" muscle glycogen. Accepting the well documented theory that low muscle glycogen causes fatigue, one can see how glycogen sparing can potentially extend performance or allow for an acceleration late in a race.

"Glycogen sparing" is widely recognized as a potential ergogenic strategy, but the best training techniques to maximize fatty acid oxidation are still hotly debated.

Conventional Training for "Fat Burning"

Some time ago, people noticed; at certain intensities individuals rely more on the different metabolic pathways and substrates to synthesize ATP. Low intensity exercise can be sustained through oxidation of fatty acids, while high intensity exercise relies more on the oxidation of carbohydrate from blood glucose and/or glycogen. This principle became the foundation upon which weight loss programs and training regimes were built. The birth of the "Fat Burning Zone" was inevitable.

The exact intensity range of the "Zone" is still being debated - research indicates that it varies from individual to individual (Achten et al., 2002, Carey, 2009). The general consensus is that it occurs at a relatively low intensity, near 60-65% of VO2max. Somewhere down the line, coaches, physiologists or athletes took this data and assumed since maximal fat oxidation rate (MFO) occurs at low intensities, it is best to train at low intensities for long durations to increase one's ability to oxidize fatty acids. Example: Maffentone

One Problem: The Paucity of Data

Unfortunately, for the long-slow-distance enthusiast, these notion that training in the "zone" improves his fat oxidation rate more so than training at a higher intensity is not supported by the literature. When it comes to fat oxidation, it doesn't seem to matter whether an athlete trains at or above the intensity that corresponds to MFO (Alkahtani et al., 2013). In 2004, Achten and Jeukendrup wrote:
To the best of our knowledge no studies have investigated
which training is most effective in inducing changes in fat metabolism.
The effects of intensity and duration of training programs on
fat oxidation should be investigated to predict such changes.

The Physiological Model:

If we work our way through the physiological model, it becomes more apparent how one could potentially improve his maximal fat oxidation rate - increasing his aerobic capacity (VO2max). Suppose a marathon runner trains hard and improves his VO2max. For simplicity's sake let's assume his running economy (RE) and fractional utilization at marathon pace remain the same after training.

Increasing VO2max alone will not change the percentage of VO2max or the percentage of lactate threshold/pH threshold the marathon can be run at - so it will not change substrate utilization (indicated by respiratory exchange ratio). Increasing VO2max will translate to a faster pace at any given percentage of max, again - assuming RE doe snot change. Increased running speeds require more ATP, and O2 consumption. Where does the additional ATP come from? It comes from increased oxidation of substrate (carbohydrate and fatty acid).

Further, if an athlete increases VO2max and then can run the same pace at a lower percentage of VO2max, he is likely to rely less on carbohydrate and more on fatty acid metabolism.

Yes, increased VO2max ↔ increased fatty acid oxidation → increased performance.

What about "Aerobic Base?"

For too long, aerobic metabolism has been misunderstood - looked at as a separate entity from anaerobic metabolism. But, it's more complicated than that. Metabolic pathways are interlinked. Coaches and athletes should take notice and training should follow suit. There are no training zones to maximize separate and distinct variables because there are no separate and distinct variables. The bulk of the research shows that low intensity training is not likely to be the best way to improve one's ability to oxidize fat. And high intensity training (above the second ventilatory threshold) does not diminish one's ability to oxidize fat at the expense of carbohydrate - rather, it's the opposite.

Instead of long slow distance, perhaps there is more merit to high intensity training, aimed at increasing VO2max:  Further reading -,


  • Increasing fatty acid oxidation can spare muscle glycogen, potentially improving endurance performance
  • There is an ongoing debate over what type/intensity of training best improves fatty acid oxidation rate
  • Increasing VO2max also increases maximal rate fatty oxidation
  • Low intensity exercise may not not be the best way to maximize fatty acid oxidation. Perhaps athletes should focus on high intensity training (above the second ventilatory threshold). High carbohydrate diets and/or carbohydrate supplementation may enhance high intensity exercise performance.


Achten, J., Gleeson, M., & Jeukendrup, A. E. (2002). Determination of the exercise intensity that elicits maximal fat oxidation. Medicine and Science in Sports and Exercise, 34(1), 92–97.

Achten, J., & Jeukendrup, A. E. (2004). Optimizing fat oxidation through exercise and diet. Nutrition, 20(7), 716–727.

Alkahtani, S. A., King, N. A., Hills, A. P., & Byrne, N. M. (2013). Effect of interval training intensity on fat oxidation, blood lactate and the rate of perceived exertion in obese men. SpringerPlus, 2, 532.

Carey, D. G. (2009). Quantifying differences in the “fat burning” zone and the aerobic zone: implications for training. Journal of Strength and Conditioning Research / National Strength & Conditioning Association, 23(7), 2090–2095.

Margaria, R., Cerretelli, P., Aghemo, P., & Sassi, G. (1963). Energy cost of running. Journal of Applied Physiology, 18(2), 367–370.


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